A conventional motor generator system, as utilized for example in the aerospace industry, includes a brushless synchronous machine that generates multi-phase AC power from a rotating shaft, e.g., coupled to a gas turbine engine, and DC excitation. In addition to operating in a generator mode, the brushless synchronous machine operates as a starter (motor) to start the aircraft engine. Following a successful engine start the system initiates the generator mode.
Conventionally, motor controllers for applications requiring a controlled torque use discrete sensors to determine rotor position in a rotating machine. This technique, however, increases system complexity and decreases system reliability. The electric machine must have a sensor built in or attached mechanically to the rotor. Interfaces and wiring must be added for control (excitation) and feedback signals between the controller and the sensor. Typical sensors include resolvers, encoders, and the like. The location of the rotating machine could be far from the controller, creating the need for unwanted extra wiring in the system.
A conventional motor control system having a position sensor is shown in FIG. 1A. The primary components of the system include a power source 110, a controller 120, a motor generator 130 and a speed/position sensor 140. The controller 120 includes inverter control 126 that receives signals from the sensor 140 (e.g., speed/rotor position) and the motor generator 130 (e.g., current, voltage). These signals are used to control the main inverter 122 and exciter inverter 124, thereby providing a conventional closed loop system to regulate the current as a function of the speed of the motor generator 130, as will be appreciated by those skilled in the art.
FIG. 1B illustrates a block diagram of a sensorless system. As is apparent from the block diagram, the sensor and related signals to the controller 120 are absent. Those skilled in the art will appreciate that this requires the controller 120 to process the rotor position/speed of the motor generator 130 to allow closed loop current regulation or to execute certain control functions (e.g., current control) or operate in an open loop mode.
Sensorless motor control techniques can increase system reliability and eliminate the need for extra wiring in the system. In addition these techniques eliminate the need for a discrete position sensor and also reduce the system cost. A sensorless motor control technique is a more flexible/adaptable solution for a motor drive system than one that relies on a separate position sensor. It is particularly valuable for an aircraft system where increased reliability and reduction of weight (e.g., through elimination of the sensor and additional wiring) are extremely important.
Motor controller applications in systems with existing electrical machines can use a sensorless motor control scheme. For example, sensorless control systems are advantageous in retrofit applications, where a sensor and appropriate wiring may be unavailable and not easily installed. Some of these systems have synchronous generators that can be used as a motor generator but they do not have discrete sensors. Additional applications for this technique include motor controllers in the environmental control systems, electric power systems, industrial drive systems, and the like.
One known sensorless technique for determining rotor position observes back EMF voltage, which may be defined as Eemf=k w sin α, where k is a constant, w is the angular speed of the motor, and α is the electrical phase angle of the rotor.
However, although observing back EMF to derive rotor position does not rely on a dedicated sensor, such a technique is not well suited for providing initial position sensing at standstill (zero back EMF) or low speed ranges (low signal to noise ratio), which is necessary at start-up under high load torque of the motor generator.